1. Technical Field
The present invention relates to an image forming apparatus.
2. Description of Related Arts
In an image forming apparatus according to an electrophotographic system, in order to fix a toner image formed on a sheet, a fixing apparatus is used. The fixing apparatus is constituted by a heating roller and a pressing roller which comes in pressure contact with the heating roller, and the fixing apparatus conveys a sheet (i.e. a paper sheet) to a fixing nip portion formed between both the rollers, and performs heating and pressing treatment, thereby fixing a toner image onto the sheet.
Generally, the temperature control of the fixing apparatus controls electric power supply to a heater to heat the heating roller such that the detected temperature of a temperature sensor to detect the surface temperature of the heating roller becomes a predetermined target control temperature. As a temperature control method during printing, an ON/OFF control or a PID control (Proportional-Integral-Derivative Control) is used. When the detected temperature is lower than the target control temperature, the ON/OFF control supplies a fixed electric power to the heater. In the ON/OFF control, temperature ripples occur with a phenomena in which overshoot and undershoot occur alternately. That is, after the surface temperature of the heating roller has been heated to the target control temperature, overshoot occurs with a phenomena that the temperature continues to rise more excessively. Thereafter, at the time of returning the rising temperature to the target control temperature, next, undershoot occurs with a phenomena reverse to the phenomena of the overshoot. A PID control controls electric power supply by switching it in multi-stages in accordance with a temperature difference between a detected temperature and a target control temperature or a change of a detected temperature. In the case where printing is started and fixing is started successively for a plurality of sheets, alternatively, in the case where the pressing roller is not heated sufficiently up to its inside at the time of starting printing and an amount of heat of the hating roller shifts to the pressing roller, since heat supply by a heater runs short, the temperature of the heating roller falls at the initial stage. As compared with the ON/OFF control, in the PID control, there is a problem that such a temperature fall at the initial stage at the time of starting printing becomes large.
Patent Literature 1 (Japanese unexamined Patent Publication No. 2010-15130) discloses an image forming apparatus. In the image forming apparatus, when a returning operation is performed to raise the temperature of a heating roller from a standby state of being heated at a standstill to a temperature capable of passing a sheet, electric power is supplied by an ON/OFF control during a predetermined time period from the time when the heating roller has started rotating. Successively, after the predetermined time period has elapsed, the returning operation is switched to a heating operation by a PID control.
In the technique disclosed by Patent Literature 1, the control is switched from an ON/OFF control to a PID control in accordance with the elapsed time from the rotation start of the heating roller. Accordingly, in the case where switching timing is too late, a problem arises in that undershoot becomes larger at the initial stage. On the other hand, in the case where switching timing is too early, another problem arises in that overshoot becomes larger.
The present invention has been achieved in view of the above-mentioned circumstances, and an object of the present invention is to provide an image forming apparatus which enables the temperature of a fixing member to converge to a target control temperature early at the time of starting an image forming operation and can minimize undershoot, overshoot, and temperature ripples.